Not sure what the technical term for this is, but the head settles in use

[WobblyHand]

I've noticed that even locking the z-axis gibs on my PM25MV, the head drops a tad after use. By a tad, I mean 0.01 mm. This is after 5-10 minutes. For what I do, most of the time it doesn't matter, but sometimes it does. Here is an example of what happened.

You can see slots in the surface. I didn't change the z axis, it was locked. The R8 collet was tight. The quill was locked. What can cause this? What is the technical term for this effect? Is there a fix for it? For what it is worth, it doesn't always happen, but it happens frequently enough that it's annoying, and can spoil the appearance of a piece.

 

[mmcmdl]

Heat buildup ? .003 isn't much if the spindle , tool , and part is getting hot . Why not rough it out and finish with a skim cut ?

 

[WobblyHand]

Heat buildup ? .003 isn't much if the spindle , tool , and part is getting hot . Why not rough it out and finish with a skim cut ?

That's possible. Hadn't thought of that. The z-axis DRO value changed, and the slot was created, (non intentionally) even though everything was locked. Probably was heat built up in my under powered mill. Seems there is more drop if there is a heavier load on the cutter. A skim cut doesn't seem to drop or cause the last digit to twinkle very much compared to a heavier one.

That picture was of a practice piece made of 6061. (Wanted to learn how to make this piece on something less costly.) The final piece was made of brass, and the same surface came out fine. Back flow valve piston replacement for my irrigation system.

 

[RJSakowski]

It sounds like spindle growth due to thermal expansion. Spindle bearing friction causes the spindle to warm up and its length increases slightly. Higher spindle speeds will enhance the effect. Many machinists will ruin run their machines for ten or so minutes before starting any actual machining to minimize this effect.

 

[pdentrem]

Many of our machines have to be running awhile to get all the clearance etc eliminated. Our Landis cylindrical grinder has to run at least an hour before attempting a job, especially if the finish tolerance is 0.0002” over 6”. Surface grinders same time, runs all day to keep the heat in the spindle.
Pierre

 

[mmcmdl]

I used to run a start up program on the cnc lathes each morning to get everything up to temp . The Makino would hold a tenth all day with no problem . Damn good lathe .

 

[WobblyHand]

It sounds like spindle growth due to thermal expansion. Spindle bearing friction causes the spindle to warm up and its length increases slightly. Higher spindle speeds will enhance the effect. Many machinists will ruin their machines for ten or so minutes before starting any actual machining to minimize this effect.

Don't doubt that there is growth due to thermal expansion. How would my z-axis DRO know that the spindle got longer? My z-axis is showing the drop as well. This is the change I see over time running the mill. Well, now that I say this, I need to check the sign and what it means! Better go downstairs and check.

The z-axis goes more negative over time, meaning the cutter goes into the piece. Some is the spindle, but that won't be shown by the DRO. Guess my question was about the z-axis DRO. However, I've just been taught about spindle growth, as well.

 

[RJSakowski]

Where is the z axis DRO attached? The mill DRO's that I have installed are attached at the bottom of the spindle so the DRO is separated from the work by only the length of the milling cutter.

 

[mmcmdl]

Not sure of the exact number , but I saw on the history channel that a railroad track across the country entire expanded and contracted a couple of miles due to temp changes . I don't know if this is true , seems like a whole bunch to me .

 

[WobblyHand]

Where is the z axis DRO attached? The mill DRO's that I have installed are attached at the bottom of the spindle so the DRO is separated from the work by only the length of the milling cutter.

Good question. Don't know, exactly, but I will check. If mine is in that location then seeing the negative delta makes sense, since the head is expanding due to heat along with the spindle. Well, learned something new today, that's good!

The z-axis attachment is level with the bottom of the head casting. However, some of the spindle is below that. Ok, what I see makes some sense. Sounds like I should be using my roughers more to lighten the load a bit and use the better mills for finishing.